U.S. Pat. 4,577,427, dated Mar. 25, 1986 and issued to the assignee herein shows an electromagnetic display device of the foregoing type wherein the housing includes a pair of high remanence magnetically reversible cores for driving the magnet and the rotor. The cores are oppositely magnetized and drive a rotor magnet which has its magnetic axis in the plane of the rotor between a pair of limiting positions in each of which a contrasting face of the rotor is displayed in the viewing direction.
The patented device is quite useful but it has been found that a more economical device can be provided by using a single high remanence reversible stator core having its forward free end located on a line parallel to the viewing direction extending rearwardly from the rotation axis, to drive a rotor magnet having its polar axis perpendicular to the plane of the rotor. In this way a more economical device is obtained using only one drive core instead of two. Moreover, a simple magnetic drive is obtained because the rotor magnet is polarized perpendicular to the viewing direction, the single core may drive the rotor between limiting positions which may be, if desired, considerably greater than 180.degree. apart measured in the angle of rotation of the rotor between limiting positions.
Accordingly it is an object of the invention to provide an electromagnetic rotor of thin flat shape pivoted about a central axis in the plane of the device which is driven by a single high remanence core having its forward free end located near a line parallel to the viewing direction and passing through the rotary axis and where the rotor has a permanent magnet magnetized perpendicular to the median plane of the rotor. The magnet is located assymetrically relative to the rotor axis to provide the required drive torque when the core is switched.
In a preferred form of the invention, the magnet is made thin and flat to conform to the flat shape of the rotor element as a whole.
In a preferred form of the invention the thin generally flat magnet has a length dimension greater than its width with the length dimension generally parallel to the rotary axis. This arrangement takes advantage of the fact that the main flux of a thin flat magnet is developed adjacent the edges. Accordingly the magnetic effects are enhanced by providing the magnet as a long thin shape and the flux along each of the long edges is used to assist the drive of the rotor by the core.
The thin, generally flat magnet, as discussed, is preferably inserted as part of the middle layer of a rotor formed as a three layer laminate or `sandwich`, the rotor being constructed as defined in Pat. No. 3,871,945 dated Mar. 18, 1975 and 3,953,244 dated Apr. 27, 1976 and held by the assignee of this application.
Such thin generally flat magnet may be made in various ways but for the best combination of high coercivity and remanence with sufficient structural qualities for handling during construction, it has been found best to use a plastic bonded ferrite which may be injection molded to the thin shape desired for the flat shape and the laminated construction.
It is an object of a preferred form of the invention to provide a stator core whose forward free end is rearward of the rotor and aligned with the line from the axis parallel to the viewing direction. The core is designed and located to project into the path of the rotor. The edge of the rotor is thus provided with a cut-out to allow the rotor to pass the projecting core end. This allows the core end to be brought closer to the locus of the magnet in its travel from one limiting position to its other, improving the magnetic torque.
It is an object of the invention using the rotor with the edge cut-out to provide rotor stops, supported on the housing, which define the limiting position for the rotor, which stops are disposed on opposite sides of the rotor axis. The stops are so located that each respectively acts on that portion of the rotor which is symmetrically disposed across the rotor axis from the cut-out. In this arrangement the cut-out in part of the range of rotation of the device, allows the disk, in its movement between limiting positions, to clear that stop which is not in use. This effectively enlarges the permissable rotation angle of the rotor while allowing convenient arrangement of the stops. This cut-out also provides clearance for the forward-extending portion of the stator core.